In the construction of hot pressing equipment there are compromises involving pressure and temperature capability, sample volume capacity, and the adjustment of pressure and temperature process parameters over time. Equipment designs are selected to maximize the resolution on those parameters which are not critical to the particular purpose, but no single design can be suitable to all purposes.
One major subclass of equipment is based on the work of Bridgman and is exemplified in commercial application by the seminal work at General Electric in the field of diamond synthesis. The inventions disclosed in U.S. Pat. Nos. 2,941,241, 2,941,242, 2,941,243, and 2,941,248 demonstrate equipment in which high pressures and temperatures are produced in a small volume. The load bearing components in these structures are constructed of metal alloys and tungsten carbide, with seal components being constructed of ceramic materials. This construction provides excellent pressure capability but has limited temperature and volume capability due to the low hot strength of the materials of construction. Similar equipment is demonstrated in U.S. Pat. Nos. 3,350,743 and 4,097,208.
A second subclass of equipment operates at lower pressures, with larger sample volumes and occasionally higher temperatures. In this subclass the materials of construction are based upon graphite. Cylindrical punch and die assemblies are commonly used as well as more complicated geometries, as shown in U.S. Pat. No. 4,102,679. A recent variation involving a quasi-isostatic pressure technique is shown in U.S. Pat. No. 4,853,178.
In addition to the steady-state capability for holding a given combination of pressure and temperature, U.S. Pat. No. No. 3,488,153 teaches the desirability of rapid heating by capacitive discharge. In addition U.S. Pat. No. 4,097,274 teaches that the capability for rapid heating and short dwell time at peak temperature is desirable (e.g. in the manufacture of polycrystalline diamond articles).
An overview of hot pressing in general with a section pertinent to direct resistance heating is presented in the ASM Metals Handbook, Ninth Edition (volume 7, Powder Metallurgy).
A salient feature of the current state of the art is the limitation of power input to the sample and the resultant limitation on heating rates. Power input densities are typically less than 10.sup.6 watts/cm.sup.3 and heating rates are less than 200 degrees C./sec except for capacitive discharge systems.
There is an inherent disadvantage with low input power densities in that power dissipated in the sample is lost to the surrounding press structure before the peak process temperature is reached. This results in decreased efficiency, longer heating times and additional thermal loading of the press structure.
Hot pressing as currently practiced is carried out at or near conditions of thermal equilibrium. Indeed, the ASM Metals Handbook, Ninth Edition teaches that in order to achieve acceptable results, a hot pressing system must allow thermal equilibrium conditions (volume 7, Powder Metallurgy, p. 503). Aside from the heating and cooling segments of the process cycle, there is typically a prescribed process dwell temperature that is held for a period of time lasting from a few seconds to several hours. The direct resistance heated process in particular tends to minimize temperature gradients within the sample since heating occurs throughout the volume of the sample. Even for mixtures of dielectric and electrically conductive materials in which only one component is being directly heated, the gradients are generally small due to the slow heating rates and the short thermal path into the dielectric phase. Once the process reaches its dwell temperature, thermal equilibrium between the phases is quickly established.
The prior art methods cited above note the desirability of power efficiency, rapid heating capability and high temperature capability but do not effectively achieve such. The means by which these goals are pursued are presented in the context of material resistances, applied voltages and the resultant currents and power dissipations. This is a limited approach. A comprehensive design of a direct resistance heated hot pressing process must include a complete electrical design that addresses not only the resistive nature of the press structure and sample, but the capacitive and inductive elements as well. Full consideration must be given to the power supply and hot press structure as an electrical system if the performance is to be optimized with respect to rapid heating, power efficiency and peak temperature capability. Although rapid heating has been shown to be desirable, and capacitive discharge has been demonstrated as a viable means of achieving rapid heating, capacitor banks are often attached to equipment that was designed for steady state operation, as in U.S. Pat. No. 3,488,153. Accordingly, the prior art has not been successful in developing high speed, low inductance heated press structures, and substantial room for improvement remains in these areas.